The invention relates to optical communication, and, more particularly, to a full range offset correction technique for coherent optical orthogonal frequency-division multiplexing OFDM Systems.
Orthogonal frequency division multiplexing (OFDM) is a prevailing multiplexing technique in digital mobile communication systems and starts to rise in optical communication systems. It provides flexible multiple access and multiplexing/demultiplexing ability in a low-cost and spectral efficient way. In the OFDM modulation, the data symbols are divided into multiple orthogonal narrow-bandwidth subcarriers and transmitted simultaneously. Due to its feasibility of digital dispersion compensation and narrow-bandwidth subcarriers, it can be very resistible to multi-path fading and chromatic dispersion. This makes it outstand among other modulation schemes.
However, the detection can encounter dramatic difficulty if the frequency of the local oscillator (LO) has an offset from the incoming signal, which may come from Doppler effects, channel fading, or the instability of the frequency synthesizers in transmitters or receivers. The tolerance of the frequency offset is much smaller than the subcarrier spacing, makes it difficult to implement in the environment of large frequency offset. For example, in the coherent optical OFDM (CO-OFDM) fiber communication system, the local laser, as a LO, can be shifted by a few GHz due to temperature variation. Fortunately, the detection problem caused by frequency offset can be perfectly corrected if the frequency offset amount is precisely known.
Accordingly, there is a need for a quick wide searching range frequency offset estimator coherent optical OFDM systems.